1. Field of the Invention
The present invention relates to a neutron absorption detector, such as a neutron absorption detector that detects neutrons by use of the Cherenkov effect.
2. Discussion of Related Art
The detection of thermal neutron neutrons is well known. For example, thermal neutron detection is usually accomplished with 3He tubes that are routinely incorporated into commercial instruments. 3He tubes typically are filled with gas at pressures in excess of two atmospheres. Transportation of these tubes by air requires a waiver issued by the Department of Transportation; without this waiver, the tubes must be delivered by ground transportation. A tube's structure comprises a cylinder filled with gas in which a thin wire is strung axially under tension. This wire structure is susceptible to vibration and this causes false counts. Detectors based on 3He tubes, with suitable moderator around them, can be made to approach 100% intrinsic efficiency.
Glass scintillators for thermal neutron detection are commercially available. Saint-Gobain markets Li-loaded silicate glass made from a recipe approximately 40 years old. In addition, PNNL (Pacific Northwest National Laboratory) has produced Li-loaded glass fibers for neutron detection, and has licensed the process to Nucsafe, LLC. Nucsafe manufactures the fiber and uses it in both portable and fixed neutron detectors. A basic difficulty associated with Li-loaded glass scintillators is that the neutron response is not well distinguishable from the gamma response when there is a high gamma flux.
The production of light by the glass envelopes of photomultipliers is well known. It is believed that such light is caused by scintillation processes in the glass and Cherenkov light. Such light has been deemed as noise and so steps have been taken in the past to reduce the effect of such light in low-noise photomultipliers. It should be noted that Cherenkov light is emitted when a charged particle, such as an electron or a positron, moves faster than the speed of light in a medium. Gamma and x rays of sufficient energy can produce Cherenkov light indirectly by liberating electrons from atoms in the medium (Compton scattering and photoelectric effect) and by generating positrons (pair production). For example, Cherenkov light is the source of the blue glow surrounding the core of swimming pool reactors and spent fuel in storage ponds. The condition for the production of Cherenkov light is given byβn>1,  Eq. (1)wherein n is the index of refraction of the medium in which the particle is traveling and β is equal to the ratio v/c, wherein v is the speed of the particle and c is the speed of light in vacuum.
There are instances where the detection of Cherenkov light is desirable in high-energy physics applications. Such detectors are routinely used for muons and very high-energy (>1 GeV) particles. Such detectors include RICH (ring imaging Cherenkov) detectors which are made with glass gels of various index of refraction butted together and ordered according to index of refraction so that the Cherenkov cone developed in each section of gel is superimposed on all the others to form a ring of light that is indicative of the energy of the particle passing through the assembly.
There also exist water Cherenkov detectors, such as those at the Sudbury Neutrino Observatory (SNO) and the Kamioka Observatory (Super-Kamiokande) that are used for the detection of Cherenkov light caused by the interactions of neutrinos with electrons or nucleons. These interactions result in high-energy electrons that produce Cherenkov light.
In another water Cherenkov detector, the water includes a neutron absorbing material. When neutrons pass through the water, they are captured by the neutron absorbing material resulting in the emission of prompt gamma rays. Such gamma rays then energize electrons to such an extent that the electrons produce Cherenkov light within the water.
For portal monitoring, both non-spectroscopic plastic scintillator and spectroscopic NaI detectors are commercially available. Cherenkov detectors will not replace NaI or any other spectroscopic device; however, several embodiments of the present invention may provide a more effective detector for gamma rays above 300 keV, while being insensitive to most medical isotopes. Notably omitted from the “insensitive” list are positron emitters producing 511 keV annihilation gamma rays. Consequently, radiation from patients recently examined by Positron Emission Tomography (PET) could be expected to be detectable by Cherenkov light.